According to the U.S. Census Bureau, the county has a total area of 2,436 square miles (6,310 km2), of which 2,403 square miles (6,220 km2) is land and 33 square miles (85 km2) (1.4%) is water.[5] One of the world's tallest Douglas fir trees ever recorded was in the town of Mineral within Lewis County, attaining a height of 120 metres (390 ft).[6]

Lewis County is arguably the most conservative county in western Washington. It is significantly more Republican than the counties it borders, with the possible exception of Yakima County. Unlike much of western Washington, it has a strong tinge of social conservatism. In 2000 George W. Bush received over 60% of the county's vote. In 2008 John McCain defeated Barack Obama by over 18 percent—his only victory in a county west of the Cascades. McCain lost all the neighboring counties except Yakima. The Republican candidate has won by over 10% in every Presidential election since 1992.[7] In the last 64 years the only Democratic candidate to win the county was Lyndon B. Johnson.[8] As part of Washington's Third Congressional District it is represented by Republican Congresswoman Jaime Herrera Beutler.

There were 26,306 households out of which 31.60% had children under the age of 18 living with them, 55.90% were married couples living together, 9.90% had a female householder with no husband present, and 29.40% were non-families. 24.00% of all households were made up of individuals and 11.20% had someone living alone who was 65 years of age or older. The average household size was 2.57 and the average family size was 3.02.

In the county, the population was spread out with 26.50% under the age of 18, 8.20% from 18 to 24, 25.20% from 25 to 44, 24.50% from 45 to 64, and 15.50% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 98.30 males. For every 100 females age 18 and over, there were 95.40 males.

The median income for a household in the county was $35,511, and the median income for a family was $41,105. Males had a median income of $35,714 versus $23,453 for females. The per capita income for the county was $17,082. About 10.40% of families and 14.00% of the population were below the poverty line, including 18.60% of those under age 18 and 9.40% of those age 65 or over.

Ware, Louisa Jackson (1865). Daily journal of Louisa Jackson, 1865.Available online through the Washington State Library's Classics in Washington History collection Daily diary for the entire year of 1865, recording the details of pioneer life in Washington Territory from the perspective of a 12-year old girl who was part of a prominent Lewis County family. Brief entries document the activities of running the farm and the number of visitors and immigrants that stopped at the Jackson home